Master cylinder for hydraulic braking systems



April 27,1943. MILLEI'QQ i 2,317,473

MASTER CYLINDER FOR HYDRAULIC BRAKING SYSTEMS Original Filed Feb.'2, 1940 Patented Apr. 27, 1943 MASTER CYLINDER FOR HYDRAULIC BRAK- ING SYSTEMS Kay Miller, Rockford, 111., assignor to The Atwood Vacuum Machine Company, Rockford, 111., a copartnership composed of Seth B. Atwood and James T. Atwood Original application February 2, 1940, Serial No.

Divided and this application October 2, 1941, Serial No. 413,335

14 Claims.

This application is a division of my copending application Serial No. 316,896, filed February 2, 1940.

This invention relates to hydrau ic braking systems and has particular reference in an improved master piston and cylinder mechanism therefor.

The aim of certain improvements in hydraulic braking systems in recent years has been to provide for quick take-up of the slack in the system and obtain the desired braking force with lower brake pedal pressure, whereby to eliminate the use of boosters or any other power type braking means and secure greater safety in addition to easier and generally more satisfactory operation. However, so far as I am aware, thi object has not been attained and such devices as have been proposed have not been practical, if operative at all, and they were moreover too complicated and expensive, and as a result have not been adopted commercially. It is, therefore, the principal object of my invention to provide a master cylinder having a compound type piston so constructed and so arranged with relation to the braking system and the brake fluid reservoir that the kind of operation desired is obtained in a highly practical and eillcient manner and without complicating the construction or adding to the cost appreciably, if any.

The invention is illustrated in the accompanying drawing, in which- Figure 1 is a longitudinal section through the master cylinder and reservoir equipped with a compound piston constructed in accordance with my invention, the cylinder being illustrated as connected with a brake system and arranged to have its piston operated by a pedal operated lever, and

Fig. 2 is a view of the main parts of Fig. l in moved positions.

The same reference numerals are applied to corresponding parts of these views.

Referring to the drawing, 5 is the master cylinder having conduits 6 extending from the discharge end thereof to the various brake cylinders 1 associated with the brake shoes 8 and the brake drums 9 of the wheels of .the vehicle, the shoes 8 being operable under hydraulic fluid pressure from the conduits '6 by pistons Ill in the usual way, and arranged to be retracted by suitable spring means This portion of the brake system is shown diagrammatically inasmuch as the present invention relates primarily to the master cylinder 5 and its compound piston 12, except, of course, in so far as these elements 5 and I2 cooperate with the rest of the brake system, like the braking fluid reservoir I3 and a pedal or otherwise manually operated lever l4 in new and patentable combinations. The lever 14 is shown as pivoted at I5 and arranged to be moved manually in a counter-clockwise direction, as indicated by the arrow 16, and to be returned by suitable spring means, indicated at IT. The lever l4 may on the other hand be operated by suitable power means where so desired although, as will soon appear, the compound piston 12 enables such easy manual operation that so-called boosters or other power means are actually not necessary. The lever I4 is shown as connected by means of a link 18 with the outer end of the piston rod I9 of the piston 12.

The cylinder 5 has a bore 2| of uniform diameter from end to end. The reservoir 13, which is shown as formed integral with the wall of the cylinder, but which, of course, may be a. separate unit, has open communication with the bore 20 through a large port 22 and a small port 23. A stufiing box 26 provides a fluid-tight seal for the rear end of the bore 20 around the rod 19.

I A plug 2'] is threaded in and closes the front end of the bore 20 and carries the nipples t which the conduits 28 for the various brake cylinders are connected. A seat 29 provided on the plug 21 has the usual large return valve 30 seating thereon under action of a coiled compression spring 3|. The latter engages an annular seat 35a on the return valve, as clearly appears in the drawing, and functions to return the piston 12 to retracted position after application of the brakes. The valve 30 carrie an outlet valve 32, which is held closed under light pressure by means of a spring 33.

The compound piston l2 shown comprises an outer large tubular piston I20 and an inner piston l2d having a close working fit in the axial bore 34 of the outer piston, the annular grooves 40 and 41 in the outer piston l2c are interconnected by one or more radial passages 42 and the groove 43 in the inner piston 12d communicates normally with the groove 41 in the outer piston, whereby to establish communication between the bore 20 in the cylinder 5 and the reservoir 13 in the following manner: From reservoir l3 through port 22 to groove 40, from groove 40 through passage 42 to groove 4|, and from groove 4| through passages 44, 45, and 46 to the bore 20. The washer 35 provides a limiting abutment for the outer piston against which the spring 31 holds the piston, the spring being backed up by the ring 38 entered in the groove 39. It will be noticed that the bore is of uniform diameter from end to end, the ring 38 providing a sumcient shoulder to limit return movement of the compound piston li. The outer piston lic has one -or more longitudinal passages 48 extending from the rear end thereof forwardly to an annular internal groove 48 communicating with an annular external groove 50 in the inner piston lid. An annular shoulder 5| defining one side of the groove Ill normally seals the annular groove 49 by closing the front end of the axial bore 34 in the outer-piston lie, but when the inner piston lid moves forwardly more or less independently of the outer piston lie, as in Fig. 2, the end of the axial bore 34 is opened into communication with the compression cylinder 41 and the outer piston lie is accordingly free to creep forward with but in lagging relation to the inner piston, the pressure in front and behind the outer piston being continuously balanced through passage 48. In other words, the outer piston is thereafter in floating relation to the inner piston and is therefore inoperative, the inner piston being the only one capable of causing further displacement of oil from the bore ill.

. In operation, the small port 23 connects the compression cylinder 41 with the reservoir ll when the compound piston Ii is in the retracted position shown in Fig. 1. Under these conditions the large port ii is in open communication with the bore ill as follows: It communicates with the front end of the annular external groove 40 in the outer piston lie, and the external groove '43 in the inner piston lid is in communication with the internal groove 4| in the outer piston lie so that communication with the bore ill is established through passages 44-46. However as soon as the compound piston ii is moved forward, the compression cylinder 41 is cut of! from communication with the reservoir II by the outer piston lic covering the small port 23, and the piston li accordingly discharges oil, or whatever other braking fluid is used, from the compression cylinder 41 through the outlet valve and thence through the conduits 6 into the brake cylin-,

ders 1, thus forcing the pistons Ill outwardly to apply the brake shoes 8 against the resistance of their springs ll. When the compound piston li has been moved enough to take up all or most of the slack in the braking mechanism of the car, there is suflicient back pressure in the compression cylinder 41 to overcome the spring 31 and any further movement of the piston rod l9 will. therefore, cause the inner piston lid to move forwardly independently of the outer piston lie in the manner shown in Fig. 2. when that occurs, the passage 48 serves to bypass oil so as to balance the pressure on opposite sides of the outer piston lie, and the spring 31 thereupon becomes eifective to move the outer piston. lie forward with the inner piston lid. The outer piston, in other'words, creeps forward with the inner piston just so long as the inner piston moves but remains in lagging relation thereto. The inner piston is, therefore, the only one capable of causing further displacement of oil from the bore 20. The pedal pressure with this construction is lighter than with certain other constructions heretofore devised, because the spring 31 does not work in opposition to the pedal during the application of high pressure to the brakes. When the brake pedal pressure previously appliedto the rod I9 is released, the spring l1 returns the lever l4 and the spring 3| returns the compound piston Ii toward normal retracted position. In

this operation the fluid pressure in the compression cylinder 41 is, of course. immediately relieved, so that there is nothing to interfere with the outer piston lic moving forward relative to the inner piston lid under action of the spring 31 to a point where it strikes the washer 3i, and, as a result, the compound pistonis free to return, because the oil returns to the reservoir l3 through the same ports and passages communicating with the port ii through which it entered the bore il, grooves 4i and 43 being in communication at this time. The functioning of the compound piston Ii, it should be clear, is entirely automatic in so far as the point at which the inner piston lid will commence to move forwardly independently of the outer piston lie is concerned, because regardless of wear on the brake shoe linings, the compound piston will move forwardly as a whole until all or most of the slack in the mechanism has been taken up and a predetermined back pressure is developed in the conduits land cylinder 41 suflicient to overbalance the spring 31, whereupon the inner piston lid is free to move forwardly independ. ently of the outer piston lic.

It is believed the foregoing description conveys a good understanding of the objects and advantages of my invention. The appended claims have been drawn to cover all legitimate modifications and adaptations.

I claim:

1, In a hydraulic brake system, a master cylinder having one or more conduits for conducting operating fluid to and from the discharge end of a bore therein, a main piston having a working fit in said bore, manually operable brake operating means, a secondary piston directly operable by the latter of smaller diameter than the main piston and slidable in a second bore in said main piston so as to displace fluid from the discharge end of the first mentioned bore, a reservoir for operating fluid, means providing restricted communication between the reservoir and said first bore at-a point in front of the main piston in its retracted position, other means providing communication between the reservoir and said first bore behind said main piston, said means including means for cutting oil! such communication upon movement of the secondary piston forwardly relative to the main piston, means for establishing communication through the main piston between the space in the bore in front of said piston and the space in the bore behind said piston, said means including means normally cutting off such communication but opening such communication upon forward movement of the secondary piston relative to the main piston, and spring means operatively connecting the main piston with the secondary piston to move forwardly together but permit forward movement of the secondary piston relative to the main piston, said spring means being yieldable to permit forward movement of the secondary piston relative to the main piston when the operating fluid offers a predetermined resistance to forward movement of said main piston, the said pistons constituting the sole means for displacing oper ating fluid from said first bore.

2. In a hydraulic brake system, a master cylinder having one or more conduits for conducting operating fluid to and from the discharge end of a bore therein, a main piston having a working fit in said bore, manually operable brake operating means, a secondary piston directly operable by the latter of smaller diameter than the main piston and slidable in a second bore in said main piston so as to displace fluid from the discharge end of the first mentioned bore, a coiled compression spring surrounding the secondary piston and compressible between it and the main piston for moving the main piston forwardly with the secondary piston, a reservoir for operating fluid, intercommunicating ports and passages in the pistons and cylinder normally establishing communication between the reservoir and the first bore behind the pistons, such communication being maintained only so long as the pistons are movable together as a unit, a washer on the end of the secondary piston for limiting forward movement of the main piston relative to the secondary piston under action of the compression spring, a passage in the main piston for'establishing communication between the opposite ends of the first bore, said passage being arranged to be closed by the washer so as to be opened upon forward movement of the secondary piston relative to the-main piston, and a coiled compression spring having engagement at one end on said washer and at its other end in the discharge end of said first bore to return said piston after forward movement thereof.

3. In a hydraulic brake system, a master cylinder having one or more conduits for conducting operating fiuid to and from the discharge end of a bore therein, a main piston having a working fit in said bore, manually operable brake operating means, a secondary piston directly operable by the latter of smaller diameter than the main piston and slidable in a second bore in said main piston so as to displace fluid from the discharge end of the first mentioned bore, spring means operatively connecting the secondary piston with the main piston to move forwardly together, the said pistons constituting the sole means for displacing operating fluid from said first bore, a fluid reservoir, means forming a passage between the reservoir and the first bore behind the main piston partly through the main piston and partly through the secondary piston, said means including valve means for closing said passage upon forward movement of the secondary piston relative to the main piston, and means forming another passage between the space in the bore in front of the pistons and the space in the bore behind said pistons through said pistons, said means including valve means arranged to open said passage upon forward movement of the secondary piston relative to the main piston.

4. In a hydraulic brake system, a master cylinder having one or more conduits for conducting operating fluid to and from the discharge end of a bore therein, a main piston having a working fit in said bore, manually operable brake operating means, a secondary piston directly operable by the latter of smaller diameter than the main piston and slidable in a second bore in said main piston so as to displace fluid from the discharge end of the first mentioned bore, spring means operatively connecting the secondary piston with the main piston to move forwardly together, the said pistons constituting the sole means for displacing operating fluid from said bore, a fluid reservoir, means forming a passage between the reservoir and the first bore behind the main piston partly through the main piston and partly through the secondary piston, said means including valve means for closing said passage upon forward movement of the secondary piston relative to the main piston, and means forming another passage between the space in the bore in front of the pistons and the space in the bore behind said pistons through said pistons, said means including valve means arranged to open said passage upon forward movement of the secondary piston relative to the mainpiston, the valve means being formed by relatively movable portions on the main piston and secondary piston.

5. In a hydraulic brake system, a master cylinder having one or more conduits for connecting operating fluid to and from the discharge end of a boretherein, a main piston having a working fit in said bore, manually operable brake operating means, a secondary piston directly operable by the latter of smaller diameter than the main piston and slidable in a second bore in said main piston so as to displace fluid from the discharge end of the first mentioned bore, a coiled compression spring surrounding the secondary piston and compressible between it and the main piston for moving the main piston forwardly with the secondary piston, the said pistons constituting the sole means for displacing operating fluid from said first bore, a reservoir for operating fluid communicating with the discharge end of said first bore through a port in the wall of the cylinder closed by the main piston in its initial forward movement, means comprising intercommunicating ports and passages in the pistons and cylinder normally establishing communication between the reservoir and the first bore behind the pistons, such communication being maintained only so long as the pistons are movable together as a unit, said main piston having a passage provided therein to establish communication between the opposite ends of the first bore, and valve means closing said passage arranged to be opened only upon forward movement of the secondary piston relative to the main piston.

6. In a hydraulic brake system, a master cylinder having one or more conduits for conducting operating fluid to and from the discharge end of a bore therein, a main piston having a working fit in said bore, manually operable brake operating means, a secondary piston directly operable by the latter of smaller diameter than the main piston and slidable in a second bore in said main piston so as to displace fluid from the discharge end of the first mentioned bore, a coiled compression spring surrounding the secondary piston and compressible between it and the main piston for moving the main piston forwardly with the secondary piston, the said pistons constituting the sole means for displacing operating fluid from said first bore, a reservoir for operating fluid communicating with the discharge end of said first bore through a port in the wall of the cylinder closed by the main piston in its initial forward movement, means comprising intercommunicating ports and passages in the pistons and cylinder normally establishthe main piston, said valve means being formed by relatively movable portions on the main piston and secondary piston.

7. In a hydraulic brake system, a master cylinder having one or more conduits for conducting operating fluid to and from the discharge end of a bore therein, a main piston having a working fit in said bore, manually operable brake operating means, a secondary piston directly operable by the latter of smaller diameter than the main piston and slidable in a second bore in said main piston so as to displace fluid from the discharge end of the first mentioned bore, a coiled compression spring surrounding the secondary piston and compressible between it and the main piston for moving the main piston forwardly with the secondary piston, the said pistons constituting the sole means for displacing operating fluid from said first bore, a reservoir for operating fluid communicating with the discharge end of said first bore through a port in the wallet the cylinder closed by the main piston in its initial forward movement, and means comprising ports and passages in the pistons and a communicating port in the wall of the cylinder opening into the reservoir providing open communication between the reservoir and the first bore behind said pistons so long as the pistons are movable together as a unit but arranged to simultaneously close oil? that communication and establish communication between the opposite ends of the first bore when the secondary piston moves forwardly relative to the main piston.

8. A device of the character described, com prising, in combination, a cylinder, a main piston having a close working fit in said cylinder, a secondary piston extending into one end of the cylinder and slidable in a longitudinal bore extending through the main piston, the other end ofsaid cylinder having a discharge port, means positively limiting movement of the secondary piston relative to the main piston in a return direction, spring means resisting movement of the secondary piston relative to the main piston in the forward direction, said cylinder being sub stantially filled with hydraulic working fluid, a reservoir containing hydraulic working fluid, the reservoir having restricted commmunication with the cylinder in front of the main piston in the retracted position of the latter, said cylinder hav ing a port intermediate the ends of the main piston communicating with said reservoir, passages provided in said main piston and secondary piston for flow of fluid between the last mentioned port and the space in said cylinder behind the main piston, valve means including relatively slidable portions on the main piston and secondary piston to shut oflf said fluid flow upon movement of the secondary piston forwardly relative to the main piston, a passage provided in the main piston for fluid flow between the opposite ends of said cylinder through said piston, and valve means including relatively slidable portions on the main piston and secondary piston for shutting oif said flow except when the secondary piston has moved forwardly relative to the main piston.

9. A device of the character described, comprising, in combination, a cylinder, a main piston having a close working fit in said. cylinder, a secondary piston extending into one end of the cylinder and slidable in a longitudinal bore extending through the main piston, the other end of said cylinder having a discharge port, means positively limiting movement of the secondary I piston relative to the main piston in a return direction, spring means resisting movement of the secondary piston relative to the main piston in the forward direction, said cylinder being substantially filled with hydraulic working fluid. a reservoir containing hydraulic working fluid. said cylinder having a port intermediate the ends of the main piston communicating with said reservoir, passages provided in said main piston and secondary piston for flow of fluid between the last mentioned port and the space in said cylinder behind the main piston, valve means including relatively slidable portions on the main piston and secondary piston to shut off said fluid flow upon movement of the secondary piston forwardly relative to the main piston, a passage provided in the main piston for fluid flow between the opposite ends oi said cylinder through said piston, and valve means including relatively slidable portions on the main piston and secondary piston for shutting ofl said flow except when the secondary piston has moved forwardly relative to the main piston.

10. A device of the character described, comprising, in combination, a cylinder, a main piston having a close working fit in said cylinder, a secondary piston extending into one end of the cylinder and slidable in a longitudinal bore extending through the main piston, the other end of said cylinder having a discharge port, a fluid return valve closing said port, a compression spring for holding said valve seated arranged to be compressed by forward movement of the secondary piston, a spring check valve in the return valve permitting discharge of fluid under pressure from the cylinder, means positively limiting movement of the secondary piston relative to the main piston in a return direction, spring means resisting movement of the secondary piston relative to the main piston in the forward direction, said cylinder being substantially filled with hydraulic working fluid, a reservoir containing hydraulic working fluid, the reservoir having restricted communication with the cylinder in front of the main piston in the retracted position of the latter, said cylinder having a port intermediate the ends of the main piston communicating with said reservoir, passages provided in said main piston and secondary piston for flow of fluid between the last mentioned port and the space in said cylinder behind the main piston, valve means including relatively slidable portions on the main piston and secondary piston to shut off said fluid flow upon movement of the secondary piston forwardly relative to the main piston, a passage provided in the main piston for fluid flow between the opposite ends of said cylinder through said piston, and valve means including relatively slidable portions on the main piston and secondary piston for shutting ofl said flow except when the secondary piston has moved forwardly relative to the main piston.

11. A device of the character described, comprising, in combination, a cylinder, a main piston having a close working fit in said cylinder, a secondary piston extending into one end of the cylinder and slidable in a longitudinal bore extending through the main piston, the other end of said cylinder having a discharge port, a fluid return valve closing said port, a compression spring for holding said valve seated arranged to be compressed by forward movement of the secondary piston, a spring check valve in the return valve permitting discharge of fluid under pressure from the cylinder, means positively limiting movement of the secondary piston relative to the main piston in a return direction, spring means resisting movement of the secondary piston relative to the main piston in the forward direction,

said cylinder being substantially filled with hydraulic working fluid, a reservoir containing hydraulic working fluid, said cylinder having a port intermediate the ends of the main piston communicating with said reservoir, passages provided in said main piston and secondary piston for flow of fluid between the last mentioned port and the space in said cylinder behind the main piston, valve means including relatively slidable portions on the main piston and secondary piston to shut ofi said fluid flow upon movement of the secondary piston forwardly relative to the main piston, a passage provided in the main pis ton for fluid flow between the opposite ends of said cylinder through said piston, and valve means including relatively slidable portions on the main piston and secondary piston for shutting oiI said flow except when the secondary piston has moved forwardly relative to the main piston.

12. A compound piston mechanism of the character described for operation in a cylinder, comprising a reciprocable rod one end portion of which forms a secondary piston of small diameter for operation in the cylinder, a sleeve-like main piston of relatively large diameter having a close working fit internally on said secondary piston and a close working fit externally in said cylinder, a shoulder on the inner end of said rod limiting movement of the same relative to the main piston in one direction, a coiled compression spring surrounding the rod and acting between a shoulder on the rod and the adjacent end of the main piston to resist movement of the rod relative to said main piston in the opposite direclimiting movement of the same relative to the main piston in one direction, a coiled compression spring surrounding the rod and acting between a shoulder'on the rod and the adjacent end of the main' piston to resist movement or the rod relative to said main piston in the opposite direction, the main piston having an internal annular groove and an external annular groove interconnected by a passage, said rod having an external annular groove and a longitudinal pas sage interconnected by a radial passage, the longitudinal passage communicating with one end of the cylinder through another radial passage provided in said rod, the annular groove in the rod registering with the annular groove in the main piston but being movable out of register therewith when the rod is moved relative to said main piston against the resistance of said compression spring, a longitudinal passage provided in the main piston to establish communication between the opposite ends of the cylinder, an internal annular groove in one end portion of the main piston remote from the first mentioned internal annular groove and communicating with one end of said passage, and an external annular groove in the end portion of the rod spaced from the end thereof in register with the last mentioned internal groove and arranged in the spring restrained movement of the rod relative to the main piston to establish communication between the adjacent end of the cylinder and the tion, the main piston having an internal annular groove and an external annular groove interconnected by a passage, said rod having an external annular groove and a longitudinal passage interconnected by a radial passage, the longitudinal passage communicating with one end of the cylinder through another radial passage provided in said rod, the annular groove in the rod registering with the annular groove in the main piston but being movable out of register therewith when the rod is moved relative to said main piston against the resistance of said compression spring, a longitudinal passage provided in the main piston to establish communication between the opposite ends of the cylinder, and a valve on said rod for shutting off such communication, said valve being opened when the rod is moved relative to the main piston against the resistance of said compression spring, the cylinder having a radial port provided therein communicating with the external annular groove in the main piston throughout the range oimovement of said piston.

13. A compound piston mechanism of the character described for operation in a cylinder, comprising a reciprocable rod one end portion of which forms a secondary piston of small diameter for operation in the cylinder, a sleeve-like main piston of relatively large diameter having a close working fit internally on said secondary piston and 'a close working flt externally in said cylinder, a shoulder on the inner end of said rod last named material annular groove.

14. In a hydraulic brake system, a master cylinder having one or more conduits for conducting operating fluid to and from the discharge end of a bore therein, a main piston having a working fit in said bore, manually operable brake operating means, a secondary piston directly operable by the latter of smaller diameter than the main piston and slidable in a second bore in said main piston so as to displace fluid from the discharge end of the first mentioned bore, a coiled compression spring surrounding the secondary piston and compressible between it and the main piston for moving the main piston forwardly with the secondary piston, the said pistons constituting the sole means for displacing operating fluid from said first bore, a reservoir for operating fluid communicating with the discharge end of said first bore through a port in the wall of the cylinder closed by the main piston in its initial forward movement, means comprising ports and passages in the pistons, and a communicating port in the wall of the cylinder opening into the reservoir providing open communication-between the reservoir and the first bore behind said pistons so long as the pistons are movable together as a unit, but arranged to close ofl that communication when the secondary piston moves forwardly relative to the main piston, and other means comprising ports and passages in the pistons arranged to establish communication between the opposite ends of the first bore when the secondary piston moves forwardly relative to the main piston.

I KAY MILLER. 

